1. Field of the Invention
The present invention relates to controlling of an acousto-optic filter, and more particularly to a method and an apparatus for controlling a frequency and an amplitude of a high-frequency drive signal so as to maximize an output transmittivity for a transmission wavelength simultaneously with tuning the frequency of the high-frequency drive signal to an optical signal wavelength In the acousto-optic filter, and also to a method and an apparatus for adjusting the transmission bandwidth of an acousto-optic filter which selects light with a given wavelength from an input signal.
2. Description of the Related Art
Firstly, the description is presented in relation to the first aspect of the invention which relates to a method and an apparatus for controlling a frequency and an amplitude of a high-frequency signal (radio frequency signal) for driving an acousto-optic filter so as to maximize an output transmittivity of an optical signal.
The acousto-optic filter is a variable wavelength filter for filtering light with a wide variable wavelength range.
As an example of an acousto-optic filter, FIG. 1 shows a configuration of the acoustically-tunable optical filter disclosed in ECOC'89 15th European Conference on Optical Communication--Post-Deadline Papers, Volume 3, pages 70-73. As shown in FIG. 1, the acousto-optic filter is constituted by two, optical waveguides 106 formed by titanium diffusion on a substrate of lithium niobate (LINbO.sub.3), a first TE-TM splitter 107, a second TE-TM splitter 108, an electrode 109 and an acoustic beam region 110. An optical signal inputted from a first input terminal 102 is split by the first TE-TM splitter 107 into a TE polarization wave and a TM polarization wave which advance through two optical waveguides, respectively, and meet together at the second TE-TM splitter 108 before being outputted from the first output terminal 104. In this case, when an electric signal of a certain frequency is inputted to the electrode 109, an optical signal of a wavelength corresponding to that certain frequency undergoes a TE-TM mode conversion due to acousto-optic effects to develop in the optical waveguides within the acoustic beam region 110. Consequently, only the optical signal of such wavelength is forwarded to the second output terminal 105 and the optical signal of any other wavelengths is forwarded to the first output terminal 104. Since the wavelength of the signal that is outputted from the second output terminal 105 can be changed by changing the frequency of the electric signal, it is possible to construct a variable wavelength filter using the first input terminal 102 and the second output terminal 105 as an input and an output, respectively.
A control system of a conventional acousto-optic filter which is used for such applications as a multiple wavelength communication is shown in FIG. 2 in a block diagram. An acousto-optic filter 201 receives an optical signal of multiple wavelengths. An output optical signal of the acousto-optic filter 201 is split by a beam splitter 203 and the intensity of the output signal beam is detected by a beam intensity detector 204. The acousto-optic filter 201 is driven by a high-frequency oscillator 202 with a variable frequency but the driving frequency thereof is frequency-modulated finely or in small adjustments to an oscillation frequency of an oscillator 205. An output of the beam intensity detector 204 is synchronously detected at the oscillation frequency of the oscillator 205 by a multiplier 206 and the frequency of the high-frequency oscillator 202 is controlled by a controller 207 so that the synchronous detector output becomes zero. Here, reference is made to characteristic graphs shown in FIGS. 5A and 5B for control principles of frequencies of the high-frequency signal that drives the acousto-optic filter. An input signal beam applied to the acousto-optic filter 201 is with multiple wavelengths so that, when the frequency of the high-frequency oscillator 202 is scanned, wavelength channels are successively selected, and the output signal beam intensity changes, for example, as shown in FIG. 5A. In this case, since the driving frequency of the high-frequency oscillator 202 is finely frequency-modulated to the oscillation frequency of the oscillator 205, the output of the synchronous detector at the oscillation frequency of the oscillator 205 by the multiplier 206 has differential characteristics as shown in FIG. 5B in relation to FIG. 5A so that, by using a control method such as proportional-plus-integral control (PI control), the frequency of the high-frequency oscillator 202 can be controlled, for example, at point A of FIG. 5B through the adder 208.
When the conventional technology described above is used, the transmission wavelength of the acousto-optic filter can be tuned to the input optical signal wavelength.
However, since the amplitude of the high-frequency to drive the acousto-optic filter is not controlled, the transmittivity of the optical signal is not optimized for the overall variable width of the acousto-optic filter.
The problems existing in the above described conventional technology is overcome by the present invention which provides a method and an apparatus for controlling the frequency and the amplitude of a high-frequency drive signal so as to maximize an output transmittivity for a transmission wavelength simultaneously with tuning the frequency of the high-frequency drive signal to an optical signal wavelength in the acousto-optic filter.
Now, the second part of the description of the related art given hereinafter relates to the second aspect of the invention which provides a method and an apparatus for adjusting a transmission bandwidth of the acousto-optic filter.
Conventionally, an acousto-optic filter has been in use as a means for selecting one wavelength from a signal with multiple wavelengths or as a means for eliminating spontaneous emission optical noise from the output of an optical amplifier. When the input optical signal is modulated, even if there are differences because the spectrum width differs for each wavelength channel or there are changes with time due to device characteristic fluctuations or modulation methods, it has not been possible to cope with these differences or changes since the filter transmission bandwidth is fixed for the device length at the time of design. For this reason, the acousto-optic filter transmission bandwidths to be set during the acousto-optic filter design must match the wavelength channel having the maximum spectrum width among all the wavelength channels used in the system and also the values of such widths must be values broad enough to compensate for differences that accompany the changes with time.
During wavelength multiplex transmission, the interval between wavelength channels must be set with the above mentioned unnecessarily broad transmission widths. This presents a problem which makes it difficult to realize high-density wavelength channels. Also, when the spontaneously emitted optical noise is to be removed from the output of an optical amplifier, there is a problem with increased noise because of the unnecessarily broad transmission widths. In relation to these problems, a method for changing the transmission bandwidth of an acousto-optic filter by using an external signal described by Amnon Yariv in "Optical Waves in Crystals" Chapter 10 (1984) is known as a method to adjust the power of a high-frequency signal inputted to the filter.
In the above method, when the high-frequency signal power applied to the acousto-optic filter is made small, crosstalk increases since the transmission width broadens and the sidelobe suppression ratio deteriorates if used as a wavelength selection filter and the noise corresponding to the sidelobes is not eliminated if used to eliminate spontaneous emission optical noise from the output of an optical amplifier.
The present invention overcomes the above problems existing in the conventional technology and provides a method and an apparatus for adjusting the transmission bandwidth of the acousto-optic filter without deteriorating the sidelobe suppression ratio.